For the extra 3$ it is worth buying the RC and not having to dose unless you maintain your tank with (2) part already and have the chemicals ready on the shelf. When I use IO, I supliment the water to bring it up to where I keep my tank. I do this in the trash can I mix the water in. I test each bucket with the test kits to figure out how much of what I need. It is by no means consistant. I only do this once for each bucket. I then add what is required to each batch of water I mix up. By the time you add up the test kits, chemicals and time, $3 is a bargain.
When I use RC, I only need to do minor adjustments when I test my tank every 2 months or so. I need to do this anyways as the CA demands change in the tank and the CA reactor slows down some, etc.